1. Field of the Invention
The invention lies in the field of metal powder filled cross-linkable with plastic compositions adapted for processing by injection molding or extruding and further adapted for use under conditions involving high thermal conductivity and high electrical insulating capacity.
2. Prior Art
Heat-dissipating plastics which are also non-electrically conductive must comprise a cladding which is difficult to produce using conventional processing conditions, such as extruding or injection molding. The high shear forces occuring in such a processing can only be avoided by a cross-linking cladding composition.
A metal powder filled thermoplastic is known, for example, from the German patent application No. 34 21 105.5 (corresponding to U.S. Ser. No. 741,351 filed June 5, 1985). Here, polyolefins are synthesized in the presence of light metal powder or nonferrous metal powder by means of Ziegler catalyst and such are cross-linked at least in proximity of the metal particles. The plastic composition produced in this manner is employed either by itself or as an alloy (polyblend) with other polymers for heat elimination with simultaneous electrical insulation in electronic or electrical engineering applications. The thermoplastic composition disclosed here, which can also be employed as a "filler" for casting resins, presents difficulties when it is to be subsequently worked into a thermoplastic in comminuted form (employment temperature 140.degree. C.) Further, only a few olefins, such as, for example, ethylene can be polymerized with Ziegler catalysts to the poorly heat-stable polyethylene. In addition, the Ziegler method can only be implemented in special reactors upon exclusion of air and humidity. A simple mixing of metal grains (powders) into casting resins and a subsequent comminution of the polymerized resins likewise does not lead to suitable fillers for injection moldable thermoplastics because the insulating casing layer around individual metal particles becomes irregular during comminution, and, thus, no particularly tight packing can be achieved, or, alternatively, the insulating casing layer chips off and the filled thermoplastics do not achieve the required values of electrical insulation. Metal grains directly worked into casing resins or the like likewise only lead to thermal conductivities of about 1.5 W/mK.